An AC power supply to supply bipolar pulsed electric power is used, e.g., for a sputtering apparatus which forms a predetermined thin film on a surface of a substrate to be processed (hereinafter referred to as “a to-be-processed substrate”). As an AC power supply of this type, there is known one including a bridge circuit made up of four switching elements (MOSFETs) disposed between a DC electric power supply source and a load. When each of the switching elements of the bridge circuit is appropriately operated to apply an arbitrary pulsed voltage to a couple of targets, which are output terminals (electrodes), with the polarity of the pulsed voltage alternately reversed at a predetermined frequency, then the polarity of each of the targets is alternately switched between an anode electrode and a cathode electrode. A glow discharge is thereby generated between the anode electrode and the cathode electrode, and plasma is generated. Each of the targets is thus sputtered (see, e.g., Patent Document 1).
It is known that an arc discharge (abnormal discharge) is generated in this glow discharge for some cause or another. Because the impedance of plasma (load) rapidly becomes small when an arc discharge is generated, a rapid fall of an output voltage occurs, and consequently an output current rapidly increases. Here, if the targets are especially made of a metal such as aluminum, and the like, the targets get melted when an arc discharge of a large arc current value is locally generated between the targets. Then, the molten parts of the targets are emitted and adhered to the surface of the to-be-processed substrate. That is, particles and splashes (each being a lump having a diameter of several μm to several hundred μm) are generated, and good films cannot be formed.
Now, because an output of a DC electric power supply source generally has a constant voltage characteristic, capacitance components are more dominant than inductance components. The impedance on the plasma load side consequently becomes small (to several ohms or less in some cases) at the time of generation of an arc discharge, and the output and plasma (load) are coupled with each other to be rapidly emitted from the capacitance components to an output side. As a result, a current rise cannot efficiently be suppressed, and there is a problem in that an overcurrent flows in a short time (in several μs) (that is, a current rise rate per unit time at the time of generation of an arc discharge is high).
As one of solutions to such a problem, it is conceivable to dispose an inductor having an inductance value larger than that of plasma in at least one of positive and negative outputs from the DC electric power supply source to the bridge circuit, thereby making each output from the DC electric power supply source to have a constant-current characteristic by means of this inductor.
However, if the output from the DC electric power supply source is made to have a constant-current characteristic by the inductor, the plasma load has an inductance component, and a current rapidly flows into the plasma load at the time of reversing the polarity of each electrode. A spike-like overvoltage is consequently generated at that time. There is thus a fear that such an overvoltage induces an arc discharge.